Re: [PATCH] builtin/repack.c: invalidate MIDX only when necessary

On Tue, Aug 25, 2020 at 08:58:46AM -0700, Junio C Hamano wrote:

Jeff King [off-list ref] writes:

quoted

Does "git repack" ever remove just one pack? Obviously "git repack -ad"
or "git repack -Ad" is going to pack everything and delete the old
packs. So I think we'd want to remove a midx there.

AFAIK, the pack-redundant subcommand is used by nobody in-tree, so
nobody is doing "there are three packfiles, but all the objects in
one of them are contained in the other two, so let's remove that
redundant one".

Not AFAICT either.

quoted

And "git repack -d" I think of as deleting only loose objects that we
just packed. But I guess it could also remove a pack that has now been
made redundant? That seems like a rare case in practice, but I suppose
is possible.

Meaning it can become reality?  Yes.  Or it already can happen?  I
doubt it.

quoted

E.g., imagine we have a midx that points to packs A and B, and
git-repack deletes B. By your logic above, we need to remove the midx
because now it points to objects in B which aren't accessible. But by
deleting it, could we be deleting the only thing that mentions the
objects in A?

I _think_ the answer is "no", because we never went all-in on midx and
allowed deleting the matching .idx files for contained packs.

Yeah, it has been my assumption that that part of the design would
never change.

quoted

I'm also a little curious how bad it is to have a midx whose pack has
gone away. I guess we'd answer queries for "yes, we have this object"
even if we don't, which is bad. Though in practice we'd only delete
those packs if we have their objects elsewhere.

Hmph, object O used to be in pack A and pack B, midx points at the
copy in pack B but we deleted it because the pack is redundant.
Now, midx says O exists, but midx cannot be used to retrieve O.  We
need to ask A.idx about O to locate it.

That sounds brittle.  I am not sure our error fallback is all that
patient.

Me either. Like I said, I think that all of this is possible at least in
theory, but in practice it may be somewhat annoying in cases like these.

quoted

And the pack code is
pretty good about retrying other copies of objects that can't be
accessed. Alternatively, I wonder if the midx-loading code ought to
check that all of the constituent packs are available.

In that line of thinking, do we even need to delete midx files if one of
their packs goes away? The reading side probably ought to be able to
handle that gracefully.

But at that point, is there even a point to have that midx file that
knows about objects (so it can answer has_object()? correctly and
quickly) but does not know the correct location of half of the objects?
Instead of going directly to A.idx to locate O, we need to go to midx
to learn the location of O in B (which no longer exists), and then
fall back to it, that is a pure overhead.

Well put.

quoted

And the more interesting case is when you repack everything with "-ad"
or similar, at which point you shouldn't even need to look up what's in
the midx to see if you deleted its packs. The point of your operation is
to put it all-into-one, so you know the old midx should be discarded.

Old one, yes.  Do we need to have the new one in that case?

quoted

quoted

Teach 'git repack' to check for this by loading the MIDX, and checking
whether the to-be-removed pack is known to the MIDX. This requires a
slightly odd alternation to a test in t5319, which is explained with a
comment.

My above musings aside, this seems like an obvious improvement.

Yes.

quoted

quoted

diff --git a/builtin/repack.c b/builtin/repack.c
index 04c5ceaf7e..98fac03946 100644
--- a/builtin/repack.c
+++ b/builtin/repack.c

@@ -133,7 +133,11 @@ static void get_non_kept_pack_filenames(struct string_list *fname_list,
 static void remove_redundant_pack(const char *dir_name, const char *base_name)
 {
 	struct strbuf buf = STRBUF_INIT;
-	strbuf_addf(&buf, "%s/%s.pack", dir_name, base_name);
+	struct multi_pack_index *m = get_multi_pack_index(the_repository);
+	strbuf_addf(&buf, "%s.pack", base_name);
+	if (m && midx_contains_pack(m, buf.buf))
+		clear_midx_file(the_repository);
+	strbuf_insertf(&buf, 0, "%s/", dir_name);

Makes sense. midx_contains_pack() is a binary search, so we'll spend
O(n log n) effort deleting the packs (I wondered if this might be
accidentally quadratic over the number of packs).

And after we clear, "m" will be NULL, so we'll do it at most once. Which
is why you can get rid of the manual "midx_cleared" flag from the
preimage.

So the patch looks good to me.

Thanks.

Thanks, both. If you're ready, let's use the version in [1] for
queueing.

Thanks,
Taylor

[1]: https://lore.kernel.org/git/87a3b7a5a2f091e2a23c163a7d86effbbbedfa3a.1598371475.git.me@ttaylorr.com/ (local)